Carboxyl grafted elastomers are used as such or as co-reactants in a wide range of applications. Grafted carboxyl groups improve adhesion to various polar and non-polar substrates, compatibility with polar polymers, such as polyamides, polyurethanes, polyesters, acrylic, phenolic and epoxy resins, provide dyeing property, increase filler acceptance, etc. Reactions with carboxyl groups are employed in many crosslinking reactions, e.g., with epoxy, isocyanate, amine, hydroxyl, ester, ether, and polyvalent metal groups.
The applications of these products are extremely versatile. Carboxyl containing polymers are used in hot melt, pressure sensitive and biomedical adhesives; spray, powder and electrodepositional coatings; paints; inks; seals, packings and gaskets; for filler surface treatment; as polymeric binders and compatibilizing agents; for thermoforming and thermosetting articles; in water based systems, for detergents and surfactants preparation; as thickening agents, rheology modifiers, etc. They are used as binders for solid propellant and binders for various fibers and fillers, chip resistant coatings, electrodepositional primers, laminates and adhesives for bonding identical or different materials, such as metals, plastics, rubbers, molding compounds, textile, cement, glass, and wood in various combination and form, e.g., as sheet, fiber, wire, foam, etc.
Carboxyl containing elastomers increase green strength and adhesiveness of rubber compositions and are used in joint sealants, tire formulations, and noise and vibration damping compositions, for example in automotive applications such as an adhesive or a sealant. For such applications it is especially important that the carboxyl containing elastomers are hydrophobic to repel the water and protect the metals from corrosion.
Leighton, et al. in U.S. Pat. No. 5,066,749 assigned to National Starch and Chemical Investment Holding Corporation describes a method for preparation of hydrophobically modified polycarboxylate polymers via copolymerization of unsaturated carboxylic monomers with non-carboxylic monomers having a long hydrocarbon chain, e.g., acrylic and maleic acid with lauryl or stearyl methacrylate, in an organic solvent. Following polymerization, the polymers are extracted into an aqueous solvent and then isolated by azeotropic solvent removal.
This technique has resulted in useful elastomers, but such methods have not received general acceptance for the reason that the process is laborious and complicated.
A random copolymer formed from ethylene, alkyl acrylate and a mono alkyl half-acid ester of 1,4-butenedioic acid can be formed by continuously feeding monomers and initiator to a stirred reaction zone and continuously withdrawing a reaction mixture containing the copolymer. This is reacted for 30 minutes at 180° C. and 40,000-lb pressure. Such a polymer is Vamac sold by E. I. DuPont. The method of making such a polymer is disclosed in U.S. Pat. No. 3,904,588.
This process also requires isolation of the carboxylic polymer from the reaction mixture. The polymers obtained by this method are solids of high molecular weight and the process requires special high-pressure equipment. Another disadvantage of this process is the use of a mono alkyl half-acid ester of 1,4-butenedioic acid as a carboxylic component. It is known that the carboxyl group of such half-acid esters has lower acid strength and therefore limited reactivity as compared to the non-esterified 1,4-butenedioic acid.
It is long known that unsaturated carboxylic acids and their derivatives, such as anhydrides, can be adducted onto polymers via different techniques. For example, maleic anhydride can be added through the reaction of maleic anhydride with a diene polymer. Such a process is thoroughly described in literature, e.g., Trivedi, B. C. and Culbertson, R. M. “Maleic Anhydride” Plenum Press, NY, 1982.
Grafting of polyisoprene resin with maleic anhydride is described in U.S. Pat. No. 4,218,349 assigned to Kuraray, Ltd. The maleinized resin is used in a sulfur cured natural rubber blend to provide improved green strength of the compounds and adhesion to metal. A similar material is described in U.S. Pat. No. 4,204,046 also assigned to Kuraray, Ltd. for use as a constituent of a pressure sensitive adhesive.
Grafting of low molecular weight liquid polybutadiene resins with maleic anhydride is most widely realized on commercial scale. Such maleinized polymers are prepared by reacting polymeric resins, such as, for example, polybutadiene homopolymers or copolymers of styrene and butadiene, with a dicarboxylic acid anhydride, such as maleic anhydride. The maleinized polybutadienes are claimed to improve adhesion of elastomers to various substrates, as described in U.S. Pat. No. 5,300,569 assigned to Ricon Resins, Inc.
The aforementioned patents and publications disclose many methods for modifying elastomers via incorporation of carboxylic acid functionality. Most these methods suffer from mechanical difficulties associated with handling the increasing viscosity of the elastomer during the chemical reaction sequences. In part, the difficulty with processing of such elastomers lies in the high viscosity built upon the addition of acid group to the polymer chain. It is known that even a small number of not terminal carboxyl groups in the polymeric molecule cause interchain hydrogen bonding resulting in sharp increase of the bulk viscosity of the polymer. Therefore, the reactions leading to the formation of carboxylic polymers are usually conducted in organic solvents or in water, with the well known disadvantages associated with these techniques, for example, the necessity to remove the reaction media prior or during the use of the resin, resulting in higher energy use, slow down of the process, and VOC development. Yet another problem is the impossibility of working with high concentrations of polymer without an intolerable increase in the viscosity of the reaction mixture and resulting difficulties in agitation and heat exchange.
A solventless method for the synthesis of carboxylic polymers is described in U.S. Pat. No. 4,412,031 assigned to Nippon Zeon Co., Ltd., where a carboxyl modified rubber is obtained by the reaction of a rubber having an unsaturated carbon linkage with an organic compound having a carboxyl group and an aldehyde group in the presence of an acid catalyst, carried out in a rubber-kneading machine in the absence of a solvent. Such process requires a prolong use of sophisticated mixing equipment, like a kneading machine, and can lead to side-reactions such as gellation of the unsaturated rubber caused by the high processing temperature of about 200° C. or a chain scission caused by shear forces. Besides, the organic compounds used for this process contain an aldehyde group, hence are potentially toxic.
Another solventless method for production of saturated carboxylic polymers is described in U.S. Pat. No. 5,473,025 assigned to BASF Aktiengesellschaft. The method comprises pyrolytic cleavage of ethylene-carboxyl ester copolymers in the presence of an acid catalyst at temperatures between 150 and 250° C. Again, such a process requires high temperature, hence it is difficult to control. The method is limited to ethylene-carboxyl ester copolymers and cannot be used for unsaturated polymers that have insufficient high temperature stability.
Due to the ease of processing, anhydrides of maleic acid group are most widely used to graft or adduct unsaturated elastomers with carboxylic acid derivatives. The anhydrides are less prone to hydrogen bonding, therefore such maleinized elastomers have significantly lower viscosity than their truly carboxylic analogues. These elastomers found application as a replacement for the more difficult to prepare unsaturated carboxylic polymers.
However, the anhydride containing elastomers have some definite shortcomings. For example, maleic anhydride grafted polymers are highly moisture sensitive due to the hydrolysis of the anhydride moiety. Upon the hydrolysis of the anhydride, acid groups are produced changing physical properties of the polymer, e.g., the viscosity increases in the moisture-contact area resulting in “skinning” of the polymer surface. Besides, maleic anhydride adducted to an unsaturated polymer such as a liquid polybutadiene accelerates its oxidative crosslinking to the extent that maleinized polybutadiene resins have been used in air-drying coatings (e.g., see U.S. Pat. No. 5,552,228 assigned to Minnesota Mining and Manufacturing Co.) Therefore, storing and application of unsaturated elastomers containing maleic anhydride usually require nitrogen blanketing to provide moisture and oxygen free environment. Still another drawback of the anhydride functionality compare to the acid form is a much lower tack and adhesiveness of the polymer.
Carboxylic and carboxyl-grafted polymers can be used in a wide range of applications. Examples of such applications comprise hot-melt adhesives (e.g., U.S. Pat. No. 5,883,172); PSA (e.g., U.S. Pat. No. 5,435,879); biomedical adhesives (e.g., U.S. Pat. No. 6,139,867), pumpable adhesives (e.g., U.S. Pat. No. 5,521,248); adhesives for bonding identical or different materials (e.g., U.S. Pat. No. 5,300,569—rubber to metal adhesion, U.S. Pat. No. 5,985,392—thermoplastics to rubber adhesion, and Patent Application JP 09,299,261—polyolefin foam adhesion to steel); laminates of various substrates (e.g., U.S. Pat. No. 0,369,808—plastics, and U.S. Pat. No. 0,296,042—glass); compositions of detergents (e.g., U.S. Pat. No. 5,977,047); powder coatings (e.g., U.S. Pat. No. 5,248,400) and electrocoating (e.g., U.S. Pat. No. 4,175,018); stereolithography aid (e.g., U.S. Pat. No. 6,130,025); filler surface treatment (e.g., U.S. Pat. No. 4,496,670); compatibilizers for not miscible materials (e.g., U.S. Pat. No. 5,672,642—asphalt-polymer blends); golf ball manufacture (e.g., U.S. Pat. No. 5,824,740); sealants (e.g., U.S. Pat. No. 6,150,428); packing (e.g., U.S. Pat. No. 6,106,753); gaskets (e.g., U.S. Pat. No. 4,585,841); paint additives (e.g., U.S. Pat. No. 5,114,481); inks formulation (e.g., U.S. Pat. No. 4,137,083); inkable coating (e.g., U.S. Pat. No. 4,902,577); lubricant additives (e.g., U.S. Pat. No. 6,124,249); cosmetics (e.g., U.S. Pat. No. 5,695,747); various water based formulations (e.g., U.S. Pat. No. 4,542,791—water based sealant) among other applications. The subject matter of the instant invention is also related to U.S. Pat. Nos. 4,412,031; 4,621,127; 5,066,749; 5,473,025 and 6,166,149.
The previously identified patents and publications are hereby incorporated by reference.
Consequently, there is a need in this art for a method of preparing an unsaturated elastomer containing multiple carboxyl groups that overcomes the deficiencies of the prior art noted above. Such an elastomer can be used in crosslinkable compositions to improve the compatibility of the components of the composition, increase the green strength, adhesion and tack.